Heat the vegetable oil in a large pot over medium heat. Cook the ground meat until browned and fully cooked. Drain excess fat and set aside in a bowl.
Add the chopped onion, minced garlic, and grated ginger. Sauté until the onions become translucent and aromatic, about 7 minutes. Add in the spinach and cook until wilted, about 3 minutes.
Mix in the cooked ground meat into the pan with vegetables and stir in the mirin and hoisin sauce, cooking for another few minutes.
Cook ramen noodles according to package instructions (typically in boiling water for 4-5 minutes).
Meanwhile, whisk tahini, soy sauce, rice vinegar, chili oil, and sugar together in a bowl.
Bring oat milk and chicken broth to a soft simmer and remove from heat.
Stir the tahini and soy sauce mixture into the broth.
To assemble ramen bowls, divide noodles into bowls, ladle hot broth over noodles, add a scoop of meat and vegetable mixture, and any toppings desired.